1. Field of the Invention
The present invention relates to a sheet stacking apparatus onto which sheets are stacked and to an image forming apparatus provided with one of the sheet stacking apparatuses.
2. Description of the Related Art
As one example of sheet stacking apparatuses onto which sheets are stacked, a sheet stacking apparatus, in which an upper gripper 66 and a lower gripper 67 nip a sheet bundle and which move the sheets as shown in FIG. 15, has been heretofore used (see Japanese Patent Laid-Open No. 9-77301). Near the grippers 66 and 67, a phototransmissive sheet detecting portion 301, which can detect the sheet bundle, is provided in a manner that can be moved along with the grippers 66 and 67. The sheet detecting portion 301 includes a light emitting portion 301a, a light receiving portion 301b, and a prism 302. Light emitted by the light emitting portion 301a is diffracted by the prism 302, and then transmitted to the light receiving portion 301b. With the detection of the sheet bundle between the grippers 66 and 67, since light from the light emitting portion 301a is blocked by the sheets, the presence of the sheets can be detected.
In the apparatus described in Japanese Patent Laid-Open No. 9-77301, since the light emitting portion 301a, the light receiving portion 301b, and the prism 302 are provided near the grippers 66 and 67, the presence or absence of sheets can be monitored even when the grippers 66 and 67 are moving.
However, pencils of light rays from the light emitting portion 301a and the light receiving portion 301b must be transmitted across the range of the movement of the grippers 66 and 67, and it is also necessary to prevent the interminglement of the pencils, whereby the structure of the sheet stacking apparatus becomes complex.
Moreover, even when there is no sheet bundle between the grippers 66 and 67, another sheet sometimes enters between the upper gripper 66 and the light emitting portion 301a, and therefore the sheet is wrongly detected.